1. Field of the Invention
The present invention relates to an image forming apparatus and a fixer fluid applying roller.
2. Description of the Related Art
An electrophotographic system-based image forming apparatus finds wide application in copying machines, printers, facsimile machines, or the like equipment. In general, image formation is accomplished in the following manner. Firstly, there is prepared a photosensitive body having a photosensitive layer containing a photoconductive substance formed on the surface thereof. After the surface of the photoreceptor is electrically charged uniformly, an electrostatic latent image corresponding to image data is formed thereon through a few different image-forming process steps. The electrostatic latent image is developed into a toner image with use of a developing agent containing toner supplied from development means. The toner image is directly transferred onto a recording medium such as a paper sheet, or transferred onto an intermediate transfer medium once, and is thereafter transferred onto a recording medium. On the recording medium, the toner image is heated and pressurized, and is eventually fixed thereon by fixing means such as a development roller according to a heat fixing method.
However, the heat fixing method necessitates a heating device such as a heater which contributes to an increase in power consumption. In reality, the amount of power consumed in a heat fixing process accounts for half or more of the total power consumption entailed by all of the operations performed by the electrophotographic image forming apparatus. On the contrary, energy conservation has been addressed globally as part of efforts to prevent global warming. In the light of the widespread use of such an electrophotographic image forming apparatus, a reduction in power consumption required to fix a toner image onto a recording medium has come to be increasingly demanded on the electrophotographic image forming apparatuses. Another problem peculiar to the heat fixing method is that much time needs to be spent in fixing a multi-color toner image onto a recording medium as compared to the case of fixing a monochromatic toner image. Hence, reduction in time required to fix a multi-color toner image has also been sought after.
As an attempt to meet such demands, there has been known the use of a so-called wet fixing method using a fixer fluid containing water and a liquid which is soluble or dispersible in water and has a toner-softening or toner-swelling effect. According to the wet fixing method, toner is kept in a softened or swelled state through the application of such a fixer fluid. A toner image which is constituted by the toner in this state is attached to a recording medium and is fixed thereonto by pressurization. The wet fixing method consumes far less power than the heat fixing method, and therefore it can be said that this is an excellent fixing method in terms of energy saving. Another advantage is that the time required to fix a multi-color toner image can be reduced successfully in contrast to the heat fixing method that produces a large amount of heat. In view of the foregoing, to date various proposals have been made as to further improvement to the wet fixing method.
For example, there has been proposed a fixing apparatus in which a jet of fixer fluid ejected from a fixer fluid ejecting member having a plurality of pores is applied relatively to a toner image carried on an intermediate transfer medium or a recording medium. In this construction, the fixer fluid is applied only to a part to which toner is attached (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-109751). According to this fixing apparatus, the application of the fixer fluid helps prevent generation of curls and wrinkles in a recording medium. Moreover, in order to reduce the amount of the fixer fluid to be used, the fixer fluid is applied only to that part of a recording medium to which toner is attached, which will be referred to as “image portion”. That is, the fixer fluid is not applied to a toner-absent part of a recording medium, which will be referred to as “non-image portion”. In this construction, however, upon the fixer fluid being applied locally to a recording medium, the image portion undergoes expansion and contraction, whereas the non-image region remains unchanged. It is thus inevitable that wrinkles are generated in and around the image portion. Generation of wrinkles is particularly notable in the case of using a recording medium produced by entwining fibers dispersed in water. As a matter of course, such an inconvenience can be prevented from occurring by keeping the amount of the fixer fluid necessary to swell out toner at a minimum. However, since the minimum necessary amount of the fixer fluid is minimal, it is quite difficult to obtain a predetermined minimum value by calculation with high accuracy. Another disadvantage is that, when the fixer fluid is applied only to the toner-applied part, toner inadvertently attached to the non-image portion located near the toner-applied part by fogging remains unfixed on a recording medium, causing a smear in user's hands or clothing.
As another example, there has been proposed an image forming apparatus composed of: an intermediate transfer medium; toner image forming means for holding a toner image on the intermediate transfer medium; fixing means for fixing toner onto a recording material by applying a fixer fluid which is capable of solving or swelling the toner in such a way that the fixer fluid is attached only to the toner present on the intermediate transfer medium, with a toner-absent part kept free of the fixer fluid; and transfer means for transferring the toner image formed on the intermediate transfer medium onto the recording material (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-109747). In this image forming apparatus, although the fixer fluid is applied to the toner image formed on the intermediate transfer medium, at the time of transferring the toner image formed on the intermediate transfer medium onto the recording medium, not only the toner image but also the fixer fluid may be attached to the recording medium. Accordingly, in this image forming apparatus, just as is the case with the image forming apparatus disclosed in JP-A 2004-109751, generation of small, but appreciable wrinkles is inevitable. JP-A 2004-109747 also discloses an implementation example of the above-described image forming apparatus in which the intermediate transfer medium is subjected to a liquid-repellent treatment so as for its surface to exhibit liquid repellency against the fixer fluid. In this construction, it is unlikely that the fixer fluid is impregnated into the intermediate transfer medium. Thus, at the time of transferring the toner image formed on the intermediate transfer medium onto the recording medium, almost all of the amount of the fixer fluid is transferred onto the recording medium together with the toner image. As a consequence, the recording medium tends to curl or wrinkle more significantly.
As yet another example, there has been proposed a wet fixing method whereby fixation of toner is achieved by spraying or dropping a fixer fluid toward a yet-to-be-fixed toner image carried on a recording medium, followed by drying the recording medium. The fixer fluid in use is prepared by dispersing, in water, an organic compound which is insoluble or poorly-soluble in water and has a toner-solving or toner-swelling effect (for example, refer to Japanese Unexamined Patent Publication JP-A 7-44034 (1995)). Specific examples of the organic compound which is insoluble or poorly-soluble in water and has a toner-solving or toner-swelling effect include: an organic ester compound; an organic hydrocarbon compound; a fatty acid compound; an organic ketone compound; a halogenated hydrocarbon compound; an aldehyde compound; an ether-series compound; a heterocyclic compound; an alcohol compound; and an organic nitrogen compound. In the wet fixing method disclosed in JP-A 7-44034, however, the fixer fluid is applied to substantially the entire image forming region (toner image-presenting region) on the surface of the recording medium, regardless of a distinction between an image portion and a non-image portion. Therefore, in the case of using a paper material containing cellulose fibers as the recording medium, the recording medium tends to curl or wrinkle significantly.